Such a device would be a major breakthrough for both research and practical usage. This is not my area of expertise; therefore I hope to get some help here. AFAIK there are four possible directions: 1. Gas discharge. It seems not to be tunable and could be applied for EUV only. 2. Beta rays collision with metals. It seems not to be tunable as well and could be applied for x-ray only 3. LC-circuit. Actually this is a science fiction. Unless doing it on nano level. 4. Free electron laser. I don't have any information about it. Any comments, additions and corrections are more than welcome! Thank you in advance!